The Doppler effect is the change in frequency of a wave for an obsever moving relative to the source of the wave. The effect is often associated with an observer hearing a vehicle's horn approaching, passing and receding. The received frequency is higher than the emitted frequency during the approach, the same at the moment it passes and lower as it recedes. The Doppler effect is named after Austrian physicist Christian Doppler (1803 - 1853).
Blaine Greenhalgh from Brigham Young University explains the Doppler effect.